1. Field of the Invention
The present invention generally relates to systems having operations synchronized with a moving surface or web of material, and more particularly to a system and method for calculating the velocity, displacement and strain of a moving surface or web of material.
2. Discussion of the Related Art
There are a wide variety of known systems that involve a moving surface or web of material and the need or desire to synchronize operations with the relative movement or positions of the material. Throughout the remainder of this specification, the term "web of material" shall be construed to cover both a web of material that may carry objects thereon (such as a conveyor), as well as the surface of an object that moves relative to the measuring system. In this regard, and for purposes of this disclosure and the present invention, the term "web of material" is to be accorded its broadest reasonable interpretation, in light of the description that follows. By way of illustration, a variety of manufacturing, production, and distribution environments are known that use conveyor systems to transport items thereon. Consider, for example, the system disclosed in U.S. Pat. No. 4,198,758, which was co-invented by the inventor of the present invention. That system disclosed a chain conveyor for transporting a plurality of object carriers. It further disclosed the importance, in such systems, of tracking objects carried on or by the carrier. That system operated by using photo-emitter/detector pairs to measure the length of the chain, segment by segment. Once the length of the conveyor chain between the reference point and the object carrier equaled a known distance along the chain between the reference point (i.e., location of emitter/detector pair) and desired discharge station, then a mechanism was actuated to remove the object from the carrier.
In a significantly different environment, U.S. Pat. No. 5,578,813, assigned to the assignee of the present invention and also co-invented by the inventor of the present invention, discloses a system and method for determining relative movement between a handheld scanner and a web of material (i.e., piece of paper). Specifically, that system utilized an illumination/imaging sensor to detect relative movement between the scanner and the web of material by identifying structure-related properties of the web of material. The inherent structure-related properties of the web (such as paper fibers, or other constituents) were used for navigational purposes. Namely, to identify the navigational path of the scanner, so that the image scanned could be reconstructed electronically.
Another system is disclosed in U.S. Pat. No. 5,291,131, which discloses an apparatus for measuring the elongation of a circulating chain (elongation resulting from component wear or otherwise). The system disclosed therein uses two sensors (e.g., magnetic or optical) disposed a predetermined distance apart along the path of the circulating chain. The distance between the two indices is calculated on the basis of the calculated speed of the moving body and the time which elapse from when a first index passed a first sensor until a second index passed a second sensor. By continuing this observation over time, and comparing the calculated distances, chain elongation can be measured. U.S. Pat. Nos. 4,198,758 and 4,274,783, both entitled "Chain Measuring and Conveyor Control System" and which were coinvented by the inventor of the present invention, disclose similar techniques.
As is also known in the prior art, the print head of some ink jet printers is configured to move across the print width of a sheet of paper. The deposition of ink from the print head to the sheet of paper is closely controlled based in part on positional information of the print head. In some such printers, this positional information is obtained by stretching a transparent web of material across the span covered by the width of the sheet of paper. This web of material is passed through a slot in the print head, and contains periodically-spaced demarcation lines. An optical emitter/detector pair is disposed across the slot, and is configured to count the demarcation lines. By maintaining an accurate count of the demarcation lines, the system can maintain information related to the position of the print head along the sheet of paper. In such a system, the web of material is held stationary (in directions parallel to the direction of print head travel), while the emitter/detector pair (which is fixed to the print head) moves in relation to web of material. As is known, noise or other errors often cause the system to lose proper count of the demarcation lines, resulting in errors in the positional placement of the print head. Mechanisms are also used to track paper fed in the direction orthogonal to the print head travel.
Photocopier machines are yet another example of devices that advance a web of material (e.g., sheets of paper) through a predefined path--e.g., from a feed tray to a discharge tray. To obtain the high resolution images that are desired, the movement of the paper must be very closely controlled, as the image is transferred thereto.
In systems like those mentioned above, as well as other systems, the computation of material velocity, displacement and/or strain is often desired. For example, in such systems the stretch or shrinkage of a moving surface or web, if undetected, may inject errors into position computation. In other systems a mechanism may be provided to properly tension the web of material. Compensation of such a tensioning mechanism may be improved based upon feedback of measured or computed strain borne by the web of material. Although only a few specific examples have been presented above, it will be appreciated that it will be desirable to measure the velocity, displacement and/or strain of a web of material in a wide variety of systems, and to eliminate errors in measuring large displacements which could occur when counting relatively small displacement increments.
In addition to the systems mentioned above, other prior art systems include those disclosed in U.S. Pat. Nos. 5,089,712, 5,149,980, 5,686,720, 5,699,161 5,703,353, 5,729,008, and 5,753,908. These patents, as well as U.S. Pat. Nos. 4,198,758, 4,274,783, 5,578,813, and 5,291,131, mentioned above are hereby incorporated by reference in their entireties into the present application.
Accordingly, it is desired to provide an apparatus and method that effectively measures the velocity, displacement and/or strain on a moving web of material (or just strain of a generally stationary web of material), and that can measure large displacement of approximate length "d" without otherwise incurring errors from accumulating smaller displacement increments.